The present invention relates to apparatus and methods for remotely indicating the horizontal status of a plane. More particularly, the present invention relates to apparatus and methods for interpreting light patterns passing through a spirit level at a remote location so as to provide level status reflecting signals at a local control location. The present invention is particularly useful wherever the level status of an inaccessible plane is to be determined or wherever controls for horizontally positioning a plane are inconveniently located relative to the plane being moved.
The need for a remote indicator of the level status of a structure is frequently encountered. For instance, the level orientation of trailers or mobile homes when being parked or leveled is one such occasion. An arrangement for indicating the level status of a house trailer by means of quadrant oriented mercury contact switches is suggested in U.S. Pat. No. 3,657,695 by Birmingham. Another approach to the house trailer level indicating problem is the floating contact point detector of U.S. Pat. No. 3,660,840 by Plofchan. However the use of pendulus type mechanical devices particularly with direct electrical contact for level indicators is generally undesirable since the performance of such devices tends to degrade over long periods of usage.
It is has been well established over the years that a spirit level comprised of an arcuately formed transparent vial or closed tube filled with fluid except for a bubble therein is particularly advantageous for indicating the level orientation of a plane. Circumstances frequently render it difficult if not impossible to directly observe such spirit levels. By way of example, the level status of a trailer bed frequently cannot be easily visualized from the apparatus associated with leveling of the vehicle bed such as jack mechanisms and the like. Further, the level status of beams in building structures, submerged structures, mines and similar environments likewise renders direct observation difficult if not impossible.
It has also been recognized for some time that the light transmission, reflection or refraction characteristics associated with a fluid filled spirit level can be used to advantage in detecting the level orientation of a plane or line in that plane. An elongated light source positioned to illuminate the axial length of a spirit level with the vial positioned between this light source and photodetector devices is shown in U.S. Pat. No. 2,427,902 by Clifton et al. A similar level detecting approach using the light source and photocells oriented in a vertical plane through the axis of the vial is shown in U.S. Pat. No. 3,324,564 by Wright et al. Yet another arrangement for level indicating using the vertical plane orientation of a pair of spaced light sources and photocell detectors which control a motor drive associated circuit for automatic releveling of the spirit level is shown in U.S. Pat. No. 2,252,727 by Pepper. Pepper likewise suggests using electrically conductive fluid in the vial and embedding electrodes through the wall of the vial for actuating indicator lamps to reflect the direction of out-of-level being detected. The use of electrical contacts extending through the spirit level into the interior thereof as suggested by Pepper and others is generally undesirable since this tends to impede accurate bubble movement and requires electrically conductive fluid in the level. Further, long term reliable operation of such devices is difficult to achieve and special fabrication techniques are required.
Light beams focused through lens arrangements into a spirit level so that the light is deflected away from detectors on either end of the bubble when the structure is level is shown in Busick et al U.S. Pat. No. 2,268,017. The Busick et al system allows the unfocused light to pass through the bubble when off level to energize one of the detectors so as to enable an automatic releveling hydraulic mechanism to correct this out-of-level condition. An arrangement wherein a photocell detector is energized from light passing horizontally through the bubble or reflected from the edge of the bubble from light introduced to the end of the vial is shown in Gooley U.S. Pat. No. 3,863,067 and a similar arrangement for detecting light reflected from the meniscus end of the bubble is shown in U.S. Pat. No. 3,371,424 by Sweet. Gooley further suggests using a switching network for actuating a signalling means when the bubble is level but does not suggest indicating the direction of out-of-level or the amount of out-of-level through such switching circuitry.
From the foregoing, it is evident that the prior art devices are dependent upon the electrical conductivity qualities or the light transmission/absorption characteristics of the fluid or the reflective characteristics of the leading and trailing curved portions of the bubble. Although Busick et al relies upon light refraction characteristics of the vial and its fluid contents to deflect light away from the detectors in the absence of bubble intervention in the light path, the system shown requires critically placed focusing lenses, is subject to detection ambiguities in the region of the bubble ends, and is not well suited for use as a portable remote level sensor. Further, prior art devices such as Busick et al do not provide an indication of the direction of out-of-level using a simple three-lamp display nor are they capable of indicating discrete amounts of out-of-level as well as the direction thereof.
Accordingly there has been a continuing need for a portable device suited for use in many environments but capable of producing reliable level indicating output signals at a remote location from the line or plane for which the level condition is to be detected. Additionally, there has been a continuing need for such a device which employs a minimum number of reliable components with an effective output display for a variety of applications. Still further, there has been a continuing need for a level detector which can provide remote signals reflecting the amount and direction that a line or plane is out-of-level.